Although cortical and hippocampal GABAergic inhibitory interneurons represent only 20% of the total cortical cell population their anatomical diversity is unparalleled in the mammalian central nervous system; for example there are currently upwards of 20 acknowledged distinct members within the CA1 hippocampal formation alone. Their anatomical diversity is rich, with the morphologies of many cell types remaining local to a particular subfield, while other cell types extend wide arbor dendrites and axons that cross numerous cortical and hippocampal layers and subfields. Inhibitory interneurons often demonstrate exquisite targeting of their axons to differential postsynaptic structures. For example, axons can target selective subcellular domains (e.g. the perisomatic, axon initial segment or specific dendritic domains) to compartmentalize or time electrical activity in either a positive or negative manner. Alternatively, axons can make projections several millimeters in length, to innervate thousands of postsynaptic targets to co-ordinate the activity of both homogeneous and distributed neuronal ensembles. Moreover, each cortical interneuron subtype is unique in its proliferative history, migration during corticogenesis as well as postnatal integration into cortical circuitry. Indeed several developmentally regulated neural circuit disorders such as epilepsy, schizophrenia and autism are likely associated with deficits in the numbers and function of distinct interneuron cohorts. For all of these reasons inhibitory interneurons have recently become the intense focus of investigators drawn from a wide variety of backgrounds. Work in my Section over the last year has largely focused on two main aspects of inhibitory interneuron function: (1) We have continued our study of glutamatergic and GABAergic synaptic transmission made onto inhibitory interneurons and their downstream targets within the hippocampal formation. (2) We are also using genetic approaches to examine the embryogenesis, migration and development of specific cohorts of medial- and caudal-ganglionic eminence derived hippocampal and cortical GABAergic inhibitory interneurons. We have also commenced investigation into the developmental trajectories of identified cortical principal cells. specifically we are interested in determining whether specific pyramidal cells of deep cortical layers have a preferential wiring with specific inhibitory interneuron subtypes. We have also begun to extend our studies to include investigation into hippocampal and cortical circuits in human resected tissue. This multi-parametric approach of cortical and hippocampal development has been extremely fruitful and is a perfect example of a research strategy well suited to the intramural environment. Having the flexibility to pursue this line of research would not have been possible without the support of the NIH intramural program. Neocortical projection neurons instruct inhibitory interneuron circuit development in a lineage dependent manner In the cortex, both projection neurons (PNs) and inhibitory interneurons (INs) can be parsed into generally non-overlapping subgroups with matched laminar distributions. PNs are classified according to their efferent targets: Intratelencephalic (IT)-type project to other cortical areas and the striatum; pyramidal tract (PT)-type project to the thalamus, midbrain, brainstem, and spinal cord; and corticothalamic (CT)-type project to ipsilateral thalamic nuclei. Importantly, while IT-type PNs are found throughout the cortical depth, PT-type PNs are restricted to layer 5, and CT-type PNs to layer 6. INs can also be segregated into major groups, based on their embryonic lineage from progenitors of either the medial or caudal ganglionic eminence (MGE or CGE). MGE-derived INs include fast-spiking parvalbumin (PV)-expressing basket and axo-axonic cells, and somatostatin-expressing cells which target dendrites. Cortical CGE-derived INs are principally represented by vasoactive intestinal peptide-expressing cells that target other INs, and reelin-expressing neurogliaform cells. Similar to PNs, CGE- and MGE-derived INs are differentially biased in their laminar distributions, towards superficial layers 1 3 and deep layer 5, respectively. However, in contrast to PNs, they are initially unsorted throughout the cortical depth, and only acquire their final laminar positions over the course of the first postnatal week. These correlations in laminar profile, coupled with the delayed sorting of INs relative to PNs, suggest preferential interactions between major classes of these cell types. Indeed, multiple lines of evidence indicate that PT-type PNs, relative to IT-type, selectively influence the circuit integration and lamination of MGE-derived INs. We have now shown that IT PNs and CGE-derived INs engage similarly in parallel: they form preferential synaptic connections and loss of IT PNs disrupts the lamination and circuit integration of CGE INs during postnatal development. Specifically we show that INs derived from the CGE are selectively targeted by IT-type PNs but not neighboring PT-type PNs in deep layers. Next, we fate-switched IT PNs to PT-type throughout cortex by conditional Satb2 knockout and investigated the impact on postnatal IN development. Loss of IT PNs selectively disrupted the lamination of CGE-derived INs and their circuit integration among neighboring PNs, as reprogrammed cells targeted them at lower rates. Single cell RNA-sequencing revealed that major CGE IN subtypes were conserved, but with differential transcription of synaptic proteins and signaling molecules. Our data show that both PN class and IN embryonic lineage are important general variables during the construction of cortical circuits. The Role of AMPARs in the Maturation and Integration of Interneurons into Developing Hippocampal Microcircuits In the hippocampal CA1, CGE-derived interneurons are recruited by activation of glutamatergic synapses comprising GluA2-containing calcium-impermeable AMPARs and exert inhibitory regulalation of the local microcircuit. However, the role played by AMPARs in maturation of the developing circuit is unknown. We now demonstrate that elimination of the GluA2 subunit of AMPARs in CGE-derived interneurons, reduces spontaneous EPSC frequency coupled to a reduction in dendritic glutamatergic synapse density. Removal of GluA1&2&3 subunits in CGE-derived interneurons, almost completely eliminated sEPSCs without further reducing synapse density, but increased dendritic branching. Moreover, in GluA1-3 KOs, the number of interneurons invading the hippocampus increased in the early postnatal period but converged with WT numbers later due to increased apoptosis. However, the CCK-containing subgroup increased in number, whereas the VIP-containing subgroup decreased. Both feedforward and feedback inhibitory input onto pyramidal neurons was decreased in GluA1-3 KO. These combined anatomical, synaptic and circuit alterations, were accompanied with a wide range of behavioural abnormalities in GluA1-3 KO mice compared to GluA2 KO and WT. Thus, AMPAR subunits differentially contribute to numerous aspects of the development and maturation of CGE-derived interneurons and hippocampal circuitry that are essential for normal behaviour.